A stent placement procedure has been practiced in which a stent is put indwelling (indwelled) in a stenosed part in a living body lumen such as blood vessel, bile duct, trachea, esophagus, urethra, and other organs so as to secure the lumen. The stents for use in the stent placement procedure are classified, according to the function and placement method, into balloon-expandable stents and self-expandable stents.
Of these stents, the self-expandable stents are ordinarily formed from a shape memory alloy or the like, and can expand without any mechanical stent-expanding operation. On the other hand, the balloon-expandable stents themselves do not have an expanding function. In order to put a balloon-expandable stent indwelling in a desired stenosed part, therefore, the following procedure is carried out, for example. The stent mounted to a balloon part of a balloon catheter is disposed in the desired stenosed part, thereafter the balloon is inflated, and the stent is expanded (plastically deformed) by the expansive force of the balloon, whereby the stent is brought into close contact with the inside surface of the stenosed part and fixed in situ. In the case of placement of a balloon-expandable stent, therefore, the balloon catheter has to be inserted to the stenosed part, with the stent mounted and fixed to the balloon part. Even if the stent is securely fixed to the balloon part, however, the load arising from friction between the stent and the blood vessel or the like may cause the stent to shift on the balloon during the inserting operation. Thus, there is a risk that the stent may drop off the balloon catheter or may be disposed in a position different from the desired stenosed part.
In addition, the balloon normally used for a balloon catheter has a shape in which truncated cone-shaped tapered sections are formed on the distal and proximal sides of a straight tube section which is expanded into a hollow cylindrical shape. The balloon-expandable stent is mounted onto the outside surface of the straight tube section. At the time of insertion into the stenosed part, the stent may shift on the balloon. Therefore, there are some cases in which, although the stent does not fall off the balloon catheter, the stent shifts distally or proximally on the straight tube section so that one end portion of the stent is located on the outside surface of the tapered section of the balloon. In such a situation, the stent portion located on the tapered section is expanded only into a shape according to the expansion of the tapered section. Consequently, the stent is expanded insufficiently, which may lead to restenosis.
In order to solve such a problem, a stent delivery system has been proposed in which a part of a balloon in a folded state is clamped between stent struts so as to form a secondary projected portion, whereby the stent is fixed to the balloon. An example of this stent delivery system is disclosed in Japanese Application Publication No. 2007-135880. In this disclosed system, a stent is disposed on a balloon of a balloon catheter, the stent is diametrically contracted (crimped), and thereafter a projected portion formed by inflating the balloon is clamped between the stent struts, whereby the stent is fixed to the balloon (see FIG. 2 and paragraphs [0025], [0028], and [0029] of Japanese Application Publication No. 2007-135880).
However, in the stent delivery system disclosed in the above-mentioned application publication, it is necessary to inflate the balloon after the stent is diametrically contracted (crimped), which complicates the manufacturing process. In addition, a stress is always imposed on the balloon portion (secondary projected portion) clamped between the stent struts, so that the strength of this portion is lowered with the lapse of time. This may result in cracking of the balloon or generation of a pinhole in the balloon when the stent is expanded.